<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Em</title>
    <style>
        body {
            font-size: 20px;
        }

        .one {
            font-size: 1.5em;
        }

        .two {
            font-size: 1em;
        }
    </style>
</head>
<body>
    <div>
        <p class="one">yangkai1</p>
        <p class="tow">yangkai2</p>
        <span>yangkai3</span>
    </div>
</body>
</html>